The server isn't sending a required intermediate certificate (VeriSign Class 3 Extended Validation SSL SGC CA), so Firefox can't build a certificate chain that links to a build-in root certificate. You can import the VeriSign EV intermediate certificates from this web page (you need the second one in your case). *http://www.verisign.com/support/verisign-intermediate-ca/extended-validation-pro/index.html Copy and paste the certificate text (Secondary EV SSL Intermediate CA Certificate) to a text file VeriSignEV.cer and import the file in the Certificate Manager. *Tools > Options > Advanced : Encryption: Certificates - View Certificates That should make the error disappear.<br /> You can contact the website to inform them that they need to install that certificate.